The quadrupole ion trap, sometimes referred to as an ion store or an ion trap detector, is a well-known device for performing mass spectroscopy. A ion trap comprises a ring electrode and two coaxial end cap electrodes defining an inner volume. Each of the electrodes preferably has a hyperbolic surface, so that when appropriate AC and DC voltages are placed on the electrodes, a quadrupole trapping field is created. Typically, an ion trap is operated by introducing sample molecules into the ion trap where they are ionized. Depending on the operative trapping parameters, ions may be stably contained within the trap for relatively long periods of time. Under certain trapping conditions, a large range of masses may be simultaneously held within the trap. Various means are known for detecting ions that have been so trapped. One convenient method is to scan one or more of the trapping parameters so that ions become sequentially unstable and leave the trap where they may be detected using an electron multiplier. Another method is to use a resonance ejection technique whereby ions of consecutive masses can be scanned out of the trap and detected.
Several methods are known for ionizing sample molecules within the ion trap. Perhaps the most common method is to expose the sample to an electron beam. The impact of electrons with the sample molecules cause them to become ionized. This method is commonly referred to as electron impact ionization or "EI".
Another commonly used method of ionizing sample with an ion trap is chemical ionization or "CI". Chemical ionization involves the us of a reagent gas which is ionized, usually by EI within the trap, and allowed to react with sample molecules to form sample ions. Commonly used reagent gases include methane, isobutane, and ammonia. Chemical ionization is considered to be a "softer" ionization technique. With many samples CI produces fewer ion fragments than the EI technique, thereby simplifying mass analysis. Chemical ionization is a well known technique that is routinely used not only with quadrupole ion traps, but also with most other conventional types of mass spectrometers such as quadrupole mass filters, etc.
Most mass spectrometer systems used today include a gas chromatograph ("GC") as a sample separation and introduction device. When using a GC for this purpose, sample which elutes from the GC continuously flows into the mass spectrometer, which is set up to perform periodic mass analyses. Such analyses may, typically, be performed once a second. When performing CI experiments in such a system, a continuous flow of reagent gas is maintained.
Mass spectrometers operate at pressures that are greatly reduced below atmospheric pressure. A typical quadrupole ion trap operates at a pressure of 2.times.10.sup.-2 Torr helium, and thus requires a continuous vacuum pumping system to maintain the desired vacuum level. When operating in the chemical ionization mode, reagent gas is introduced into the ion trap at 0.1 to 100 microTorr. This pressure range is far lower than the reagent gas pressure associated with other conventional mass spectrometers which typically operate using a reagent gas pressure of 0.5 to 50 Torr. One reason the reagent gas pressure is so much lower in an ion trap mass spectrometer is that the much longer residence time of the reagent ions in the trap allows for a much longer reaction period. In other conventional types of mass spectrometers, the reagent ions are present a much shorter time and, thus, much higher concentrations of reagent gas must be used to insure that sufficient numbers of sample ions are created by CI.
In a typical commercially available ion trap configuration, the vacuum enclosure is continuously pumped at a rate of 40 to 60 liters per second. To attain the desired partial pressure of reagent gas, a volumetric reagent gas flow rate of 0.0003 to 0.3 atmosphere ml/min is required. The reagent gas is typically supplied from a pressurized bottle, with the source pressure being substantially above atmospheric pressure. A high source pressure is required for the pressure regulator to properly function and provide a stable pressure from a restricting mechanism. The large pressure differential which is placed across the gas flow control mechanism, coupled with the requirement of extremely small reagent gas flow rates has lead to the use, in prior art systems, of expensive and complicated mechanical variable restrictor valves. These valves, often called "micro leak valves", must be fabricated from high precision mechanical components. The components that are used in commercially available valves have high temperature coefficients, so that such valves are highly temperature sensitive.
In prior art reagent gas flow control systems for ion trap mass spectrometers, a solenoid valve is used in series with and downstream from the micro leak valve to turn the gas flow on and off. For example, the reagent gas might be turned off while the user of the ion trap conducts EI mass spectrometry. Later the valve may be turned on to allow the user to conduct a CI experiment. When the solenoid valve is in the off position, i.e., the reagent gas flow is turned off, pressure builds up behind the valve. Thereafter, when the solenoid valve is switched "on" to allow the reagent gas to flow, there will be a significant pressure "surge" into the ion trap due to the build up of pressure behind the solenoid valve. This large pressure surge requires that the electronics of the ion trap be turned off before, and for some time after the valve is turned on. Otherwise, the rf electronics, the electron multiplier, and the electron emission filament are subject to potential damage. This disruption causes drift and instability for a period of time.
Accordingly, it is the object of the present invention to provide a reagent gas flow control system which is less expensive and more reliable than those of the prior art.
Another object of the present invention is to provide a gas flow control system for delivering reagent gas to an ion trap mass spectrometer which can be turned off and on without causing pressure surges within the ion trap.
Yet another object of the present invention is to provide a gas flow control system which delivers a controlled low volume of gas at low pressure using simple, readily available component parts.
Still another object of the present invention is to provide a gas flow control system which is less temperature dependent, yet just as accurate as those of the prior art.